Rapidly solidified neodymium, iron, boron (Nd-Fe-B) alloys yield high performance, essentially isotropic, permanent magnet materials whose principal component is the tetragonal Nd.sub.2 Fe.sub.14 B phase. The ribbons or flakes produced by rapid solidification, i.e., melt-spinning, may be hot-worked by isostatically pressing at elevated temperatures to produce fully dense, or hot-pressed, magnets with essentially the same magnetic properties as the original ribbons. With further processing, specifically die-upsetting, magnetically aligned magnets are produced with approximately 50 percent higher remanences (B.sub.r) and approximately 200 percent higher energy products [(BH).sub.max ] compared to the hot-pressed precursor material.
The process of magnetic alignment achieved during die-upsetting has been described as a diffusion slip mechanism which requires small grain sizes, approximately 50 nanometers, and a ductile grain boundary phase. The combination of small grain size and a ductile grain boundary phase allows an orientation of the c-axis of the grains to take place along the press direction during plastic deformation. Since the c-axis is also the preferred orientation of the magnetization, the magnetic properties are enhanced along the pressed direction of the die-upset magnets.
Larger grains are deleterious to the alloy since they do not respond as well as small grains to the strains induced during die-upsetting, and accordingly remain randomly oriented, lowering the remanence and energy product of the alloy. In addition, whether aligned or not, larger grains are also associated with lower coercivities in these materials. It is therefore desirable to use lower processing temperatures and shorter times at those temperatures to limit grain growth within the alloy during the hot-working steps.
Another approach to limiting grain growth is to introduce into the alloy impurities or additives which collect in the grain boundaries. If the additive is foreign to the 2-14-1 phase inside the grain it must migrate with the boundary as the grain grows, resulting in slower grain boundary movement, and thereby slowing grain growth.
Although relatively large concentrations, i.e., approximately 10 atomic percent, of a substituent are typically required in order to have a measurable effect on the intrinsic properties of the Nd.sub.2 Fe.sub.14 B phase, much smaller additive levels, i.e., approximately 1 atomic percent, may have a substantial impact on the hard magnetic properties of a magnet. This is because the grain boundary phase, which plays a vital role in grain growth and domain wall pinning mechanisms, may be preferentially occupied by the additive creating a locally high concentration of that additive within the alloy.
Previous work has been performed on the effect of low-level additives in die-upset Nd-Fe-B magnets, where the composition of the magnets was given as Nd.sub.14 Fe.sub.77 B.sub.8 M.sub.1. This previous work concluded that gallium, wherein M=Ga, provided the largest enhancement of the coercivity, approximately 21.1 kiloOersteds, as compared to the additive-free composition, wherein M=Fe, which had the lowest coercivity of approximately 7.6 kiloOersteds. Other additives have also enhanced the coercivity but to lesser degrees. However, the remanences reported for all these magnets were lower than that of the additive-free magnet, by as much as 15 percent.
At present, the state-of-the-art concludes that additives in the Nd.sub.2 Fe.sub.14 B-type magnets must be added into the alloy at the initial melting and casting of the ingot, prior to melt-spinning and hot-working. However, it would be desirable to introduce the additive into the magnetic alloy during the hot-pressing phase, therefore permitting the additive and its concentration to be adjusted during this final step. The relatively low temperatures used in hot-working compared to either melt-spinning or sintering, probably would help limit the additive to the neodymium-rich grain boundaries where they would most likely affect grain growth and therefore coercivity.
Thus, what is needed is a method for making permanent magnetic alloys wherein the additive is introduced into the alloy prior to the hot-working steps.